The invention relates to an air nozzle arrangement in a fluidized bed boiler. The invention also relates to a grate for a fluidized bed boiler. The invention also relates to a fluidized bed boiler. The invention also relates to a method for removing coarse material from a fluidized bed boiler.
A fluidized layer refers to a layer formed by solid and granular substance, where the grains of the solid substance are in a fluidized state. The fluidized state can be achieved, for example, by fluidizing the grains by means of a fluidizing gas flow. The fluidized layer is formed in a fluidized bed reactor, which has been or is supplied with said granular solid substance. The fluidized bed reactor can be supplied with fluidizing gases from below, for fluidizing the solid substance. The fluidized layer can also be called a fluidized bed.
A fluidized bed boiler is an application of the fluidized bed. The fluidized bed boiler comprises a furnace for burning combustible material. In fluidized bed boilers, said solid substance (i.e. coarse material) comprises combustible material, burnt material, and non-combustible material, i.e. bed material, such as for example sand. In the fluidized bed boiler, the fluidized bed is formed of both the combustible material and the bed material by fluidizing with a fluidizing gas. The fluidizing gas in the fluidized bed boiler comprises oxygen. The fluidizing gas is introduced into the fluidized bed boiler via, for example, air nozzles. Heat formed in the combustion is effectively transferred to the bed material. From the bed material, heat can be recovered by a heat transfer surface, such as a heat exchanger, the heat transfer surface typically comprising heat exchanger pipes. Because the function of the heat transfer surfaces is to recover heat, heat transfer with the heat transfer surfaces of prior art is efficient. Thus, the heat transfer surface is typically clearly cooler than the bed, because the heat transfer surface is cooled by means of a heat transfer medium.
Fluidized bed boilers utilizing a bubbling fluidized bed (BFB) and a circulating fluidized bed (CFB) are known e.g. from U.S. Pat. Nos. 5,966,839A, 4,780,966A, and EP 0,028,458.
U.S. Pat. No. 5,966,839A discloses a grate assembly for a fluidized bed boiler. The grate assembly comprises means, through which cooling air is directed to a combustion chamber in the fluidized bed. The means are formed of a tubular supply channel and a substantially horizontal protective sheet at the upper end of the supply channel.
U.S. Pat. No. 4,780,966A discloses a fluidized bed comprising a sparge pipe assembly such as pipes. That invention aims to prevent unacceptable temperature differences between the upper and lower sections of the sparge pipe wall, which temperature difference results in problems of differential thermal expansion along the axis of the sparge pipe which may cause lateral buckling or distortion. In the solution, the upper section of the sparge pipe is insulated from high heat transfer, e.g. by covering the sparge pipe with a layer of denser and/or coarser particles that do not become fluidized at any fluidizing gas flow rates, and/or by protecting the upper section of the sparge pipe by a thermal insulator from the active or fluidized region of the bed.
EP 0,028,458 relates to fluidized bed boilers and burners. It provides a fluidized bed burner having a base plate with upstanding combustion air stand pipes in which at least some of the stand pipes include or have associated therewith air flow control devices. Each standpipe has its upper end blanked off and has holes in the sides. The upper ends are blanked off by an umbrella plate.
A problem in the boilers is the congealing of molten material to solid state. For example, some metals may be present in liquid state in the furnace of the fluidized bed boiler. When coarse material is removed from the boiler, heat can be recovered from the coarse material, wherein the coarse material cools down. Thus, said liquid metal solidifies. Metal can solidify, for example, in said air nozzles of the fluidized bed boiler. This can cause non-uniformness in the supply of fluidizing air.
The non-uniformnesss in the supply may impair the combustion, for example because of an insufficient supply of combustion air or excessive non-uniformness in the supply of combustion air. Furthermore, the process control may become difficult, if part of the nozzles is clogged.